1. Field of the Invention
The present invention relates to a rotating field sensor for detecting an angle that the direction of a rotating magnetic field forms with respect to a reference direction, and to an angle determination method using the rotating field sensor.
2. Description of the Related Art
In recent years, rotating field sensors have been widely used to detect the rotational position of an object in various applications such as detecting the rotational position of an automotive steering wheel. Systems using rotating field sensors are typically provided with means (for example, a magnet) for generating a rotating magnetic field whose direction rotates in response to the rotation of the object. The rotating field sensors use magnetic detection elements to detect the angle that the direction of the rotating magnetic field forms with respect to a reference direction. The rotational position of the object is thus detected.
Among known rotating field sensors is one that includes two bridge circuits (Wheatstone bridge circuits), as described in U.S. Patent Application Publication No. 2012/0053865 A1. In the rotating field sensor disclosed therein, each of the two bridge circuits includes four magnetic detection elements and outputs a signal responsive to the direction of the rotating magnetic field. The output signals of the two bridge circuits are different in phase from each other by ¼ the period of the output signals of the bridge circuits. The output signals of the two bridge circuits are used as a basis for determining an angle detection value which corresponds to the angle that the direction of the rotating magnetic field forms with respect to a reference direction.
U.S. Patent Application Publication No. 2012/0053865 A1 discloses a technique for determining an angle detection value with a reduced error by providing four detection circuits each of which includes a bridge circuit and performing an arithmetic operation using the output signals of the four detection circuits.
A known magnetic detection element for use in a bridge circuit is a magnetoresistive (MR) element that is formed by serially connecting a plurality of MR films each of which exhibits a magnetoresistive effect, as disclosed in U.S. Patent Application Publication No. 2012/0053865 A1, for example.
In the case of a rotating field sensor including two bridge circuits as described above, the determined angle detection value is no longer correct if at least one of the two bridge circuits fails.
U.S. Patent Application Publication No. 2012/0053865 A1 discloses a method including monitoring the resistances of the bridge circuits, the potentials at two output ports of each bridge circuit, the magnitude of a signal corresponding to a potential difference between the two output ports, or the like. According to the method, a bridge circuit is determined to be in failure if the monitored value exceeds a predetermined range of the normal value.
The foregoing method is able to detect such a failure of a bridge circuit that the monitored value deviates greatly from the normal value due to, for example, the occurrence of a break in any location in the bridge circuit or the occurrence of a short circuit across a magnetic detection element. Disadvantageously, however, the foregoing method is not able to detect such a failure of a bridge circuit that the monitored value deviates from the normal value by a relatively small amount. Such a failure can occur, for example, when one of the plurality of MR films connected in series to constitute an MR element is short-circuited or when the plurality of MR films become anisotropic.
U.S. Patent Application Publication No. 2012/0053865 A1 further discloses a technique for determining, in the case of a failure of one of the four bridge circuits, the angle detection value by an arithmetic operation using the output signals of two of the four bridge circuits other than the failed bridge circuit. This technique, however, is not able to detect such a failure of a bridge circuit that the monitored value deviates from the normal value by a relatively small amount, as described above. If such a failure occurs, it is not possible to identify two bridge circuits that are other than the failed bridge circuit, and consequently, it is not possible to determine a correct angle detection value.
The descriptions so far have dealt with the problem with a rotating field sensor that determines an angle detection value by using the output signals of two bridge circuits. However, the foregoing problem applies to all rotating field sensors that include a plurality of detection circuits each configured to output a signal responsive to the direction of a rotating magnetic field.